Hello Edhazer,
Unfortunately, I don't think we would be able to provide a definitive answer. You should note this forum is community-driven and not intended as a venue for customer services. We would not have access to customer accounts.
As far as I am aware, the Fido Roam charge for both Japan and China are the same (see here). Though, the current rate might have increased based on your quoted cost. According to that page, the Fido Roam fee is charged for a maximum of 20 days per billing cycle (also per destination zone; see here).
That said, you would only have access to your normal allotments when abroad. Fido Roam lets you use your data, talk and text from your existing Fido post-paid mobile plan (taken from link above). As far as I am aware, Fido does not offer any unlimited data plans so there would also be limits on data usage when abroad as well. If you have exceeded your usual data allotment, additional usage would be on a pay-per-use basis. From what I can tell, the pay-per-use data rate is the $15 per 10MB you noted. It's possible you received that message because you have gone over your normal allotment.
The Wireless Code does stipulate a cap on data roaming charges. While I don't believe that cap applies to regular Fido Roam charges, I think it would apply any roaming data overages. Though, if you accepted that message and continued to use data, I'm not sure whether that cap would apply anymore.
For a more definitive answer given your specific situation, you would need to contact customer service. I understand you have not been able to access their Live chat. They do offer a contact number you can call from abroad. They can also be contacted via Facebook, or Twitter. Those methods can be accessed via the contact page posted above.
Hope this helps 😀
Cheers
